REGARDING FEDERALLY SPONSORED RESEARCH Not Applicable.
The present invention relates generally to a fixturing or workpiece holding and clamping device and method, and in particular, to a fixturing or workpiece holding and clamping device utilizing a viscosity increase or solidification of a magnetorheological colloidal liquid or clay, as a method to secure both regular and irregular shaped workpieces for precision machining or measuring operations.
The securing of irregularly shaped delicate and brittle workpieces, such as ceramics components, glass components, and jet engine or turbine blades, for machining operations without resulting in damage to the workpiece has been found to be difficult. Typical methods of clamping through clamps or fixtures are not practical since they can cause permanent damage to the workpiece. As a result, the traditional solution to prevent damage to these workpieces includes encapsulation of a portion of the workpiece by the casting of a low melting point molten matrix material such as lead or zinc around the a portion of the workpiece, such as the airfoil section, after which the machining or measuring of the remaining portion is performed, as is seen in U.S. Pat. No. 5,947,662 to Becker et al. for xe2x80x9cSystem For Holding Thin-walled Workpiece During Machining.xe2x80x9d
Generally, this procedure involves inserting a first portion of the workpiece into a cast iron block having a cavity which is significantly larger than the workpiece itself. The molten matrix material is then poured into the cavity, surrounding and encapsulating the workpiece. After the matrix material cools and solidifies, the workpiece is secured in a fixed position for the machining or measuring operations. Upon completion of the machining or measuring operation, the matrix material is melted away from the encapsulated portion of the workpiece, leaving a finished product. This procedure, however, adds considerably to the expense of producing such workpieces, increases health and environmental risks associated with the vapors released from the molten matrix material, and fails to adequately protect the workpiece against deformation damage during the machining operations. Furthermore, such solutions cannot be applied to workpieces which are vulnerable to damage from the heating and cooling cycles associated with the addition and removal of the matrix material, or which have finished or treated surfaces which may become contaminated by residue from the molten matrix material.
Other solutions to the problem of securing irregularly shaped workpieces include the use of complex single-purpose hydraulic clamping devices such as is shown in U.S. Pat. No. 4,033,569 to Dunn for xe2x80x9cDeformation-Preventing Workpiece-Holding Fixture for Machine Tools.xe2x80x9d These devices are typically suitable for holding only a limited range of irregularly shaped objects, and operate by applying a plurality of clamping members to a number of locations on the surface of the workpiece. Application of a clamping force to only a limited number of locations along the surface of a workpiece while retaining it during the machining operations can result in the buildup of stress or damage in the workpiece from the non-uniform application of the clamping forces.
A similar solution is exemplified by U.S. Pat. No. 3,818,646 to Peterson for xe2x80x9cFixture For Holding Precisely Shaped Partsxe2x80x9d wherein an irregularly shaped workpieces, such as the thin elongated airfoil portion of a jet engine turbine blade, is secured for machining operations by a plurality of individual movable pins extending from the side wall of a clamping fixture to engage both the convex and concave surfaces of the workpiece. While increasing the number of individual movable pins extending from the side wall of the clamping fixture results in a more uniform application of clamping force to the irregularly shaped workpiece, this solution still fails to provide a completely uniform application of clamping force, and is limited to operation on workpieces having exterior surfaces with generally smooth curvature.
An alternative solution which applies a more uniform clamping pressure to the surfaces of a regular or irregular workpiece involves the use of a dry particulate material fluidized by the pressure of a gas for insertion of a workpiece, which is then substantially solidified by the application of a vacuum force or magnetic field to the dry particulate material. Examples of these types of fixturing devices may be found in U.S. Pat. No. 3,953,013 to Griffith et al. for xe2x80x9cMethod and Apparatus for Clamping A Workpiece In A Quasi-Liquid Mediumxe2x80x9d and U.S. Pat. No. 3,660,949 to Cose, Jr., for xe2x80x9cWork Holder For Irregular Shaped Workpieces.xe2x80x9d However, the use of a dry, particulate fluidizable material or quasi-liquid requires a complicated variety of associated gas injection and vacuum generating elements, as well as containment for the dry particulate fluidizable material, since an excess of fluidizing pressure can easily expel the dry, particulate material from the device.
A second alternative solution for applying a more uniform clamping pressure to the surfaces of a regular or irregular workpiece involves the use of electrofluids which respond to the presence of either alternating electric fields or a voltage difference by manifesting an apparent change in bulk viscosity. It is known that if these fluids are applied as a film over a dielectric surface, and an alternating electric field is applied to the fluid from beneath the surface, a workpiece placed on or in the electrofluid film causes the electrofluid to be energized by the electric field to secure the workpiece firmly in place. These devices, exemplified by U.S. Pat. No. 3,197,682 to Klass et al. require the application of a high voltage and potentially dangerous, three-phase current to the device, and do not permit workpieces to be immersed in the electrofluid film to any great depth, thereby limiting the clamping pressure of the device. Furthermore, electrorheological fluids are temperature sensitive, and typically have an inability to withstand water contamination, rendering them useless in machining applications wherein a machining tool is cooled by the application of water or other water-based liquid coolant to an exposed cutting surface.
Accordingly, there is a need in the industry for a self-contained fixturing or workpiece holding and clamping apparatus or device and method capable of securing both regular and irregularly shaped workpieces, such as ceramic components, glass components, and turbine blades, for machining operations with a uniform clamping force so as to reduce the stresses associated with the machining operations on the workpiece, while also being easy to use, simple to construct, and which also eliminates the risk of environmental and workpiece contamination, as well as the risk to an operator""s health from electric shock or the inhalation of harmful vapors or particles.
It is believed that an apparatus and method for immobilizing and securing both regular and irregularly shaped workpieces through the solidification or viscosity increase of a colloidal liquid, elastomer, semi-solid, or clayish magnetorheological material subjected to a magnetic field will solve many of the problems associated with traditional work holding fixtures. It is known that in the presence of an appropriate magnetic field, solid magnetizable particles in colloidal liquids, semi-solids, elastomers, or clayish materials such as jells, greases, waxes, rubbers, putty, or clay move into alignment, forming fibrous structures parallel to the applied field, significantly increasing the viscosity of the fluids and substantially decreasing the ability of the material to flow or be sheared.
A magnetizable carrier liquid or ferrofluid may be substituted for the liquid used as a carrier for the solid magnetizable particles in traditional magnetorheological liquids. While ferrofluids themselves do not solidify when subjected to an applied magnetic field, they similarly exhibit magnetic field-induced viscosity increases, and may be utilized to achieve yield stress levels significantly in excess of traditional magnetorheological liquids, as is taught by U.S. Pat. No. 5,549,837 to Ginder et al. for xe2x80x9cMagnetic Fluid-Based Magnetorheological Fluids.xe2x80x9d
The basis for the magnetorheological effect can be explained by the inter-particle forces induced by the applied magnetic field. When an external magnetic field is applied to an initially random arrangement of magnetizable particles, a magnetic moment which is approximately parallel to the applied field is induced in each particle. The force between two particles whose moments are aligned head-to-tail is attractive, promoting the formation of chains or more complicated networks of nearly contacting particles aligned along the direction of the field, significantly increasing the viscosity and essentially solidifying the material. The strength of this solidified magnetorheological material can be characterized by the yield sheer stress at which the network of aligned particles is disrupted and the particles flow. Materials having a high yield stress can sustain larger mechanical forces when solidified in the presence of a magnetic field before flowing. Magnetorheological liquids easily obtain yield stress values in excess of 5 psi in the presence of a magnetic field, and may be prepared to achieve yield stresses on the order of 20 psi as taught by U.S. Pat. No. 5,667,715 to Foister for xe2x80x9cMagnetorheological Fluids.xe2x80x9d In general, for a magnetorheological liquid, it is known that an increase in the flux density of the magnetic field to which it is subjected will result in an increase in the yield stress, i.e. an increase in viscosity which in this context is understood to mean solidification.
Among the several objects and advantages of the present invention are:
The provision of a work holding apparatus or device utilizing an increase in viscosity or solidification of a magnetorheological material to secure a workpiece;
The provision of the aforementioned work holding apparatus or device wherein the magnetorheological material has a high viscosity in the absence of a magnetic field;
The provision of the aforementioned work holding apparatus or device wherein the magnetorheological material is a semi-solid or clayish material;
The provision of the aforementioned work holding apparatus or device wherein the magnetorheological material is a liquid material;
The provision of the aforementioned work holding apparatus or device wherein the viscosity increase or solidification of the magnetorheological material is achieved by the application of a magnetic field to the magnetorheological material;
The provision of the aforementioned work holding apparatus or device wherein the workpiece is further secured by the application of a clamping force to the solidified magnetorheological material, further increasing the viscosity of the magnetorheological material;
The provision of the aforementioned work holding apparatus or device wherein a decrease in viscosity of the magnetorheological material is achieved by the removal of the magnetic field;
The provision of the aforementioned work holding apparatus or device wherein the workpiece may have either a regular or irregular shape;
The provision of the aforementioned work holding apparatus or device wherein the magnetorheological material is contained within an open-faced container;
The provision of the aforementioned work holding apparatus or device wherein the open-faced container is configured to absorb peak vibrational forces, preventing movement or climbing of the workpiece inside the work holding device;
The provision of the aforementioned work holding apparatus or device wherein the workpiece is secured for measuring or machining operations by the solidification of the magnetorheological material;
The provision of the aforementioned work holding apparatus or device wherein the magnetorheological material attenuates vibrations induced in the workpiece by the machining operations;
The provision of the aforementioned work holding apparatus or device wherein the magnetorheological material applies a uniform clamping force to the workpiece upon solidification;
The provision of the aforementioned work holding apparatus or device wherein the apparatus or device is suited for use in securing heat sensitive and non-magnetic materials;
The provision of the aforementioned work holding apparatus or device wherein the apparatus or device is suited for use in securing both metallic and non-metallic workpieces;
The provision of the aforementioned work holding apparatus or device wherein the workpiece is not subjected to a heating and cooling cycle;
The provision of the aforementioned work holding apparatus or device wherein the emission of harmful and environmentally damaging vapors or particulate matter is significantly reduced or eliminated;
The provision of the aforementioned work holding apparatus or device wherein the apparatus or device requires no external power source;
The provision of the aforementioned work holding apparatus or device wherein the apparatus or device requires no associated fluid pressure or vacuum delivery systems;
The provision of the aforementioned work holding apparatus or device wherein the apparatus or device is readily adaptable to operate as a component in an assembly line manufacturing process; and
The provision of the aforementioned work holding apparatus or device wherein the device is easy to assemble, simple to operate, and may be manufactured for a low cost.
Briefly stated, the preferred embodiment of the work holding apparatus or device of the present invention utilizes a magnetorheological material and a work holding container or fixture to secure a workpiece of either a regular or irregular shape for machining or measuring operations without damage to the workpiece. The workpiece is secured within the container, and the container or holding fixture is then positioned within a open cell containing either a liquid, semi-solid, or clayish magnetorheological material which flows or is molded around a portion of the workpiece placed within the container to conform to the surfaces of the workpiece and the open cell. The cell is located in an adjustable gap of a magnet such that a magnetic field generated by either a permanent magnet or an electromagnet will pass through the cell.
Once the workpiece is secured within the container, and partially surrounded by the magnetorheological material in the cell, a magnetic field is applied to the magnetorheological material, solidifying it to apply a uniform clamping pressure to the surfaces of the workpiece encapsulated therein. The clamping pressure may be further increased by decreasing the gap of the magnet within which the cell is placed, compressing the compressible sealing material and squeezing the solidified magnetorheological material within the cell. Under compression, the magnetic particles comprising the magnetorheological material form thick columnar structures, further increasing the viscosity or solidifying of the magnetorheological material. The solidified magnetorheological material supplies a uniform holding force to the workpiece, and allows the container or holding fixture within which the workpiece is placed to absorb any peak forces applied to the workpiece, preventing displacement thereof during a machining or measuring operation. The solidified magnetorheological material further serves to attenuate any vibrations generated in the workpiece during the machining or measuring operations. Upon completion of the machining or measuring operation, the clamping pressure is withdrawn from the cell, and the magnetic field removed, thereby allowing the magnetorheological material to revert to a less viscous state, after which the workpiece may be removed from the container or holding fixture and the device reset for a subsequent use.
In addition, the present invention also relates generally to a method for immobilizing or securing a workpiece having either a regular or irregular shape wherein a portion of the workpiece is immersed in a liquid magnetorheological material at a desired position and orientation or placed between deformable portions of a semi-solid, colloidal, elastomeric, or clayish magnetorheological material. A magnetic field is applied to the magnetorheological material to cause the viscosity of the material to substantially increase, resulting in the solidification of the magnetorheological material about the workpiece. The increase in viscosity results in the application of a uniform holding force to the surface of the workpiece. A clamping pressure applied to the solidified magnetorheological material results in an additional increase in the viscosity of the magnetorheological material, thereby increasing the uniform holding force on the surface of the workpiece, immobilizing or securing the workpiece in place. Once immobilized or secured, the workpiece is machined or measured as desired. To remove the finished workpiece, the process is reversed. First, any clamping force applied to the solidified magnetorheological material is removed. Next, the magnetic field is removed, resulting in a decrease in the viscosity of the magnetorheological material and a reversion to a rest state. Finally, the finished workpiece is removed from the magnetorheological material or from between the deformable portions.
The foregoing and other objects, features, and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings.